Spot the problem with the quoted remarks:

(1) The Oklahoma City bombing in 1995 was “something some people did.”

(2) Last month’s attack on two mosques in Christchurch, New Zealand, was “something someone did.”

(3) The 2015 massacre at a black church in Charleston, S.C., was “something someone did.”

Now imagine that a public figure with a history of making racially inflammatory remarks — someone like Representative Steve King of Iowa or, better yet, President Trump — had said any of this. (Neither of them did.) Would you not be appalled?

Of course you would. You’d be insulted by the evasiveness of the something and someone. You’d be revolted that a right-wing politician would fail to speak forcefully against the bigotries too often found among his followers and fellow travelers. You’d be disgusted by the deliberate attempt to conceal the scale of the horror, the identity of the perpetrators, and the racist ideology that motivated them.

And you’d make no allowances for the possibility that the politician in question might have merely misspoken, especially if he failed to apologize, clarify or correct himself. With political power comes rhetorical responsibility.

The bulk of Omar’s speech was devoted to preaching political empowerment for American Muslims and denouncing Islamophobia. That’s fine as far as it goes.

The problem is that the remark is foul, in exactly the same way that the hypothetical remarks listed above are foul. I live in lower Manhattan, near the 9/11 memorial and museum. No decent person can look at the portraits of the 2,983 victims of Islamist terrorists and say, by-the-by, that this was “something” that “some people did.”

The problem is also that the remarks didn’t come from just anyone. Just as Trump has repeatedly made his ethnic prejudices plain, so has Omar. She has demonized Israel, and American supporters of Israel, in terms that are unmistakably anti-Semitic. She has been reproached by fellow Democrats, claimed ignorance by way of apology, and then slurred Jews again — without apology. And despite claiming to be a champion of human rights, she has been oddly selective about the human-rights issues that elicit her outrage.

All the more so as progressives rush to her defense. Omar is not a significant figure in her own right. And the House of Representatives has never lacked for cranks, knaves, fools and bigots.

What is significant is that Omar’s defenders don’t consider her prejudices about Jews as particularly disqualifying, morally or politically, at least not when weighed against the things they like about her (and hate about her enemies). As for her views about Israel, she’s practically mainstream for her segment of the Democratic Party — a harbinger of what’s to come as the old guard of pro-Israel liberals like Majority Leader Steny Hoyer gives way to the anti-Israel wokesters typified by Alexandria Ocasio-Cortez.

What is all this reminiscent of?

Oh, right: the early days of Trump, when millions of Republican primary voters heard the candidate denounce Mexicans as drug dealers, criminals and rapists, and said to themselves, “We like that.” The central lesson of the moral collapse that followed for the G.O.P. isn’t that conservatives are a uniquely perfidious bunch. It’s that partisans of any stripe are always susceptible to demagoguery, particularly when the demagogue refuses to back down in the face of outrage. Shamelessness has a way of inspiring a following, and Omar is in the process of cornering the market on the left.

Still, let’s not be entirely negative about the congresswoman. Toward the end of her speech, she said it was vital “to make sure that we are not only holding people that we don’t like accountable: We must also hold those that we love, have shared values with, accountable.”

Those words, at least, are wise. The best thing Democrats could do now is apply them to Omar herself.

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